The expression (REF ) reveals a main difference between vertices
composed of conventional mesons and vertices containing a tetraquark and two
ordinary mesons. Indeed, in the vertices of ordinary mesons the correlation
function depends on distribution amplitudes (DAs) of one of the final-state
mesons, for example, on DAs of \(K\)  meson. The DAs of the mesons are
determined as non-local matrix elements of relevant quark fields placed
between the meson and vacuum states. In the case under discussion, it is
evident that instead of non-local matrix elements, we have \(\Pi ^{\mathrm {QCD}}(p,q)\)  that contains local matrix elements of \(K\)  meson. The reason is
that \(X_{0}\)  and interpolating current Eq. (REF ) are built of four
quark fields at the same space-time location. Substitution of this current
into the correlation function and contractions of \(c\)  and \(d\)  quark fields
yield expressions, where the remaining light quarks are sandwiched between
the \(K\)  meson and vacuum states, forming local matrix elements. In other
words, we encounter the situation when dependence of the correlation
function on the meson DAs disappears and integrals over the meson DAs reduce
to overall normalization factors. In the framework of the LCSR method such
situation is possible in the kinematical limit \(q\rightarrow 0\) , when the
light-cone expansion is replaced by the short-distant one. As a result,
instead of the expansion in terms of DAs, one gets expansion over the local
matrix elements [1]}. The limit \(q\rightarrow 0\)  is known
as the soft-meson approximation. In this approximation \(p=p^{\prime }\)  and
invariant amplitudes \(\Pi ^{\mathrm {Phys}}(p^{2})\)  and \(\Pi ^{\mathrm {OPE}}(p^{2})\)  depend only on the variable \(p^{2}\) . For our purposes a decisive
fact is the observation made in Ref. [1]}: the soft-meson
approximation and full LCSR treatment of the conventional mesons' vertices
lead to predictions which are numerically very close to each other.
